CNG is considered as a relatively clean source of energy that helps to avoid the global warming, and is expected to be more widely used as automotive fuel in place of more conventional gasoline. However, gas has a lower density than liquid or solid, and is required to be highly compressed to be stored in a limited space available in a motor vehicle or the like. To store compressed gas, a pressure vessel that can withstand a high pressure is required. Steel and aluminum are typical materials for manufacturing a pressure vessel for CNG. A metallic pressure vessel has the advantage of a high mechanical strength and a proven high reliability, but has the disadvantage of being heavy. Therefore, a heavy metallic pressure vessel can be used for a motor vehicle only at the expense of fuel economy and performance of the vehicle.
To overcome this problem, there have been proposals to manufacture a pressure vessel using composite material and thereby reduce the weight of the pressure vessel. Typically, a thin shell container (liner) made of metallic or plastic material is covered by a fiber reinforced layer impregnated with resin, and the resin is allowed to cure. See Japanese patent No. 3523802 (patent document 1), for instance.
In the proposal disclosed in FIG. 2 of patent document 1, a plastic liner is integrally molded with a metallic mouthpiece, and the assembly is covered by a fiber reinforced layer impregnated with resin, followed by the resin curing process. In such an arrangement, there is a possibility that a leak path may be formed in the interface between the mouthpiece and remaining part of the plastic liner. As a leakage through such a leak path cannot be easily repaired after the fiber reinforced layer is placed on the plastic liner, it is highly essential to ensure a reliable sealing of such a potential leak
Japanese patent laid open publication No. 2009-58111 (patent document 2) discloses an arrangement for ensuring the sealing of potential leak paths in a pressure vessel before applying a fiber reinforced resin layer. In this proposal, an O-ring is interposed between a metallic mouthpiece and a resin liner. However, as the resin liner deforms in a more pronounced way than the mouthpiece when the interior of the resin liner is pressurized, the sealing performance of the O-ring could be impaired if the deformation of the resin liner is significant.